Polyaniline is one of the extensively studied intrinsic conducting polymers and shows electrical conductivity after proton-doping or oxidation. It had been well known that polyaniline can be prepared with easiness, high yield and low cost of synthesis and also shows very high conductivity with excellent environmental stability in a conducting form. (MacDiarmid, A. G. In Conjugated polymers and Related Materials, The Interconnection of Chemical and Electronic Structure; Salaneck, W. R.; Lundstrom, I.; Ranby, B., Eds.; Oxford University Press, 1993, PP 73–98)
Poyaniline can be used in the advanced application such as secondary battery, light emitting diode, electrochromic materials and sensor because of it's excellent electrical and electrochemical and optical properties. For these applications, polyaniline should be prepared in a film or processable form. Thus, development of water-soluble polymers becomes important when commercial applications are considered. However, few solution processes have been reported because conducting polymers based on polyaniline are not soluble in common solvent due to the π-π interaction between the rigid polyaniline backbone.
In the latest 10 years, processable polyaniline that is soluble in organic solvent or water was developed by virtue of the advance in the research on solution process. Base form of polyaniline (emeraldine base) can be processed in solvent such as strong Lewis-acid, like N-methylpyrrolidone (NMP) (Tzou, K. T. and Gregory, R. V. Synth. Met. 1995, 69, 109) or concentrated sulfuric acid (Andreatta, A., Cao, Y., Chiang, J. C., Heeder, A. J. and Smith, P. Synth. Met. 1988, 26, 383). More recently, Conducting forms of polyaniline can be processed in a polar or non-polar solvent by using the acidic dopant such as dodecylbenzene sulfonic acid or camphorsulfonic acid (Cao, Y., Smith, P. and Heeder, A. J. Synth. Met. 1992, 48, 91).
The method for the preparation of water-soluble polyaniline has been also developed. Several methods such as the introduction of alkyl sulfonic acid group onto the nitrogen atom in polyaniline (Bergeron, J. Y., Chevalier, J. W. and Dao, Le H. J. Chem. Soc., Chem. Commun. 1990, 180; Chen, S. A. and Hwang, G. W. J. Am. Chem. Soc. 1994, 116, 7939), or chemical or electrochemical polymerization of anline-N-alkyl sulfonate (Bergeron, J. Y., Chevalier, J. W. and Dao, Le H. J. Chem. Soc., Chem. Commun. 1990, 180; Chen, S. A. and Hwang, G. W. J. Am. Chem. Soc. 1994, 116, 7939) were reported to prepare water-soluble polyaniline. More recently, it has been reported that water-soluble polyanline was prepared by polymerization of di-phenyl-4-sulfonic acid (DeArmitt, C., Armes, C. P., Winter, J., Uribe, F. A., Gottesfeld, J. and Mombourquette, C. Polymer 1993, 34, 158) or copolymerization of O-amino benzyl alcohol and diphenyl-4-sulfonic acid (Nguyen, M. T. and Diaz, A. F. Macromolecules 1994, 27, 7003).
Another method for the preparation of water-soluble polyaniline was synthesis of PANIs in the presence of a water-soluble polymeric acid [M. Angelopoulos, N. Patel, J. M. Shaw, N. C. Labianca and S. A. Rishton, J, Vac. Sci. Technol. B11, 2794(1993); K. Shannon, and J. E. Fernadez, J. Chem. Soc. Chem. Commum., 643(1994)].
The water-soluble polyaniline prepared by above methods can't be extracted and purified from the reaction solution and by-product after synthesis. Residual product and reagent that remains unreacted in the solution is very difficult to separate from water. Thus, the dialysis was conducted to purify the resulted polyaniline, but it takes much time and there is a doubt on the degree of purity. The excess water also should be extracted for the preparation of desired concentration of water-soluble polyaniline aqueous solution.
On the other hand, the electrochemical properties such as reversible oxidation/reduction and the stability during the potential swap cycle is very important when film or coatings of the solution-processed polyaniline are used in electrochemical applications. The most important factor that determines such properties is the ionic conductivity of the film. The diffusion rate of ions has effects on the reversibility of oxidation/reduction and the length of life. It has been reported that cycles of oxidation/reduction in N-alkyl sulfonic acid substituted polyaniline were over 100,000 in the acidic condition (Kim, E. M., Lee, M. H., Moon, B. S., Lee, C. and Rhee, S. B. J. Electrochem. Soc., 1994, 141, L26), in which the above method isn't applicable to the secondary battery or electrochromic material.
Recently, it has been reported that when the copolymer of poly(styrenesulfonic acid) and oligo ethyleneglycol acrylate was used for the dopant of polyaniline, the electrical property was enhanced (H. Tsutsumi, S. Fukuzawa, M. Ishikawa, M. Morita, and Y. Matsuda, J. Electrochem. Soc., 142, L168(1995)). In this case, the electrical property was preserved even after the 40 cycles of charging and dis-charging, but the polymerization method is very difficult and the solution process wasn't possible because the prepared polyaniline film couldn't be dissolved in solvent.